1. Field of the Invention
This invention pertains to a cryogenic system for freezing articles such as biologicals and foodstuffs. The system comprises a method and apparatus which represent an improvement over previous art in that the method enables close control of the time-temperature profile of the organic comprising article, in an efficient manner, during freezing and in that the apparatus facilitates the time-temperature profile control while enabling the freezing of a variety of different articles selectively by changing the allocation of liquid cryogen to various locations within the apparatus.
2. Background Art
The freezing of foodstuffs and biologicals requires careful consideration of the physical changes which occur in the material when it is frozen. Many biological or foodstuff materials must be frozen very rapidly to prevent the growth of damaging crystal formations which can break the cell structure of the material, resulting in destruction of the biological activity or food structure and taste characteristics. Rapid freezing is frequently obtained by direct immersion of the articles to be frozen in a cryogenic liquid. However, to economically freeze articles such as biologicals or foodstuffs using a cryogenic media, it is necessary to achieve a high degree of efficiency in use of the cryogenic media. Typically it is too expensive to completely freeze a foodstuff article solely by immersion in a cryogenic liquid. Numerous combinations of cryogenic media, including both the liquid cryogen and gaseous cryogen produced upon boiling of the liquid cryogen, have been used in attempts to obtain a high degree of efficiency.
Examples of commercially available freezing systems are those disclosed by Koach Engineering and Manufacturing Inc., Sun Valley, Calif. in undated brochures. One typical system described in a brochure comprises a liquid nitrogen immersion vaporizing system to provide fast freezing of food products. The food product is first immersed in liquid nitrogen at -320.degree. F. (-196.degree. C.) to form a frozen crust and seal the surface of the article of food. The immersion is followed by a cocurrent exposure to cold nitrogen vapor generated by the immersion freezer, to finish the task of completely freezing the article of food. Another system described in the brochure comprises three cooling zones, a "precooling zone" which uses cold nitrogen vapor, a "spray zone" wherein droplets of liquid nitrogen are applied to the food product upper surface, and a "postcooling zone" wherein cold nitrogen vapor is used to bring the product to a uniform temperature throughout.
U.S. Pat. No. 3,298,188, dated Jan. 17, 1967, to R. C. Webster, et al. describes a method and apparatus for freezing food products, including a system for diverting the cryogenic media after utilization as a liquid for further use as a vapor, to economically utilize the cryogenic media. The apparatus is designed so the food product to be frozen moves progressively up an incline as it moves through the freezer. Near the top of the incline is a spray header for spraying liquid nitrogen upon the food articles being cooled. The vapors produced at the spray header of cryogenic liquid are directed down the incline from which the food product is entering, pre-cooling the articles of food prior to their contact with the liquid nitrogen spray. Fans and baffles are used in combination with the incline to make efficient use of the pre cooling cryogen vapors flowing down the incline. In one embodiment of the invention, a liquid nitrogen bath for immersion of the food product immediately follows the spray header, to provide additional cooling.
U.S. Pat. No. 3,376,710, dated Apr. 9, 1968, to W. E. Hirtensteiner describes a food freezing apparatus for low temperature freezing of food products. The first state of the freezer comprises a direct contacting state wherein the food product is directly contacted with a liquid cryogen such as liquid nitrogen. Cold gaseous nitrogen, evolved from the first, direct contacting stage, is conveyed through a second stage of the freezer which comprises an elongated chamber where the cold, gaseous nitrogen is repeatedly circulated against the product at successive locations along and transversely of the path of advancement of the food product; the gaseous nitrogen is locally circulated and maintained at such locations (allowing relatively small net flow rate through the chamber) (Col. 1, lines 52-52). The local circulation of gaseous nitrogen is accomplished using mechanical means, a fan and baffle arrangement, in the second cooling stage of the apparatus, as shown in the drawings.
U.S. Pat. No. 3,413,818, dated Dec. 3, 1968, to J. P. Pelmulder discloses a very intricate and complex system for quick freezing of delicate cellular products. The system described includes apparatus for precooling, immersion, tempering, and postcooling of food products and biologicals. The food product is precooled in cold gas evolved from a liquid nitrogen immersion bath which follows the pre-cooling area. The food product is then immersed in the liquid nitrogen bath to form a frozen crust on the product. Subsequently, the food product is tempered in a static bath of cold nitrogen gas also evolved from the liquid bath, and finally the food product is passed countercurrent to a dynamic stream of cold nitrogen gas to further deepen the frozen crust so the product will freeze solid upon removal from the dynamic stream of gas. Air locks are used to maintain nitrogen gas pressures within the precooling and postcooling areas of the apparatus.
U.S. Pat. No. 3,427,820, dated Feb. 18, 1969, to J. Hart, discloses a food flash freezing machine comprising a tunnel structure having a conveyor belt upon which food to be frozen is moved through the tunnel. The freezing machine typically comprising a pre-cool zone through which cold gas flows, contacting the food entering the tunnel on the conveyor belt; a freezing section having means above the food conveyor belt to spray food advancing therein with a cold boiling liquid, said freezing section in communication with the pre-cool zone; and various means to draw off cold gas evolved in the tunnel structure freezing zone and to return the cold gas to other portions of the tunnel such as the pre-cool zone. Some of the machine embodiments described do not require a pre-cool zone.
U.S. Pat. No. 3,440,831 dated Apr. 29, 1969, to S. S. Thompson describes an immersion quick freeze process whereby metal parts, food items, and other such liquid immersible bodies are quick frozen to a given temperature by contacting liquid nitrogen at an expansion pressure, with a secondary refrigerant such as alcohol which is a liquid at the aforesaid temperature, to lower the temperature of the secondary refrigerant followed by immersion of the body or bodies in the secondary refrigerant while it is at the reduced temperature. More than one bath of the secondary refrigerant may be used, beginning with a warmer bath and progressing toward lower temperature baths, when it is desired to subject the body to be immersed to a more gradual temperature change.
U.S. Pat. No. 3,485,055, dated Dec. 23, 1969, to R. C. Webster, et al. discloses a continuous conveyor type cryogenic freezing apparatus for freezing food products. The product to be frozen passes in sequence through pre-cooling, liquid immersion, tempering and post-cooling zones to provide effective utilization of the cooling effect of the cryogen.
U.S. Pat. No. 4,075,869, dated Feb. 28, 1978, to R. A. Fitsall discloses an apparatus for cooling or freezing articles in which the articles are moved through a tunnel and are contacted successively by a vaporized cryogenic medium and a liquid cryogenic medium. Jets of pressurized gas are introduced into the gas contacting zone to cause turbulence in flow of vaporized cryogenic medium passing through that zone.
U.S. Pat. No. 4,229,947, dated Oct. 28, 1980, to D. J. Klee, describes a cryogenic freezer utilizing a single, centrally located blower which circulates a cryogenic refrigerant through a pair of high velocity, minimum size product contact chambers. The product contact chambers may be of variable cross section, so as to reduce the amount of refrigerant gas which must be circulated and maximize the velocity of the refrigerant to increase heat transfer.
U.S. Pat. No. 4,403,479, dated Sept. 13, 1983, to I. Rasovich discloses an apparatus for quick freezing food products including a liquid nitrogen immersion bath followed by an adjacent chamber which provides additional cooling by vapor from the liquid bath which has been drawn into the chamber. The vapor in the chamber flows cocurrently with the food product as it progresses through the apparatus. Transverse baffles are used in the chamber both above and below the conveyor belt which transports the food product through the chamber; the baffles serve to divide the tunnel into temperature zones and to direct the nitrogen vapor downward and upward over the product. The baffles are pivoted to permit product on the belt to pass the baffles.
U.S. Pat. No. 4,475,351, dated Oct. 9, 1984, to D. J. Klee describes a dual flow cryogenic tunnel freezer. The freezer comprises a plurality of individual cooling zones, each equipped with a radial fan rotating in a horizontal plane. Cryogenic liquid refrigerant is sprayed into at least one of the cooling zones in the central region of the tunnel, upwardly into the rotating fans. The vaporized refrigerant flows from the supercold zone of liquid introduction beneath the edges of partitions which separate the individual cooling zones, outwardly towards the opposite ends of the tunnel in substantially equal amounts. The vaporized refrigerant flows generally countercurrent to the incoming product to be frozen and cocurrent to the product moving toward the freezer exit.
U.S. Pat. No. 4,517,814, dated May 21, 1985, to S. O. Rothstein discloses an apparatus for continuous direct treatment of products by means of a fluid cooling medium. The apparatus comprises a chamber having means to form a liquid bed to effect direct treatment of the products in liquid cryogen contained in the bed. The liquid bed is formed by a supporting surface beneath the bed, and sidewall surfaces at opposite sides of the bed. The supporting surface has openings through which vertically directed curtains of the liquid cryogen, preferably liquid nitrogen, are emitted to create the liquid bed. The curtains are positioned at ends of the bed for retaining the bed therebetween. Nitrogen vapors formed by evaporation of the liquid nitrogen from the bath can be used for pre cooling the food product prior to immersion in the liquid bed or can be used for post-cooling in the mechanical freezer which follows the cryogenic liquid bed of the apparatus.
U S. Pat. No. 4,589,264, dated May 20, 1986, to S. Astrom discloses a tunnel freezer having a pre-cooling section and a section in which the product to be frozen is sprayed with a cryogenic liquid prior to exiting the tunnel. The freezer utilizes specially designed fans having paddle wheels, the fans being driven by a chain or belt transmission. Moreover, the tunnel is divided into compartments by partitions, to balance the flow of gaseous cryogen within the freezer. The partitions are curtains which are made of a flexible material.
Many of the above disclosures comprise known individual process steps or known individual elements of an apparatus, but in each case the combination of steps making up the overall process or method, or the combination of elements making up the total apparatus differs. In nearly all cases, the goal is to provide a more efficient method for freezing articles comprising organic contents while simultaneously preventing the formation of harmful crystal formations which would damage the structure or composition of the article being frozen, while simultaneously preventing a substantial loss of moisture which affects structure or composition of the article.
The known art provides methods of fast freezing foodstuffs, but does not provide a relatively simple and inexpensive but efficient method for controlling the temperature profile of the foodstuff during freezing, as is necessary to ensure the quality of the frozen foodstuff. For example, one of the recommended methods of fast freezing foodstuff articles is to crust freeze the article by spraying or immersing the article in liquid cryogen. Often, when the foodstuff article is not symmetrical in shape, portions of the article which are smaller in cross sectional area become brittle during crust freezing and are subject to fracture and separation from the main body of the article during handling. In addition, even when the foodstuff is symmetrical in shape, the depth of crust freezing of the foodstuff must be carefully controlled to prevent thermal cracking of the foodstuff. Since, for commonly used cryogens, at least about one half of the capacity of the liquid cryogen to remove heat from the article being frozen is available on boiling of the liquid cryogen as it contacts the foodstuff, economics demand that the depth of crust freezing during contact with the liquid cryogen be determined by the need to obtain maximum utilization of the liquid cryogen. It would, then, be advantageous to have a method and apparatus which enable not only making efficient use of the cooling capacity of the liquid and gaseous cryogen, but which also enable careful control of the crust freezing and overall time temperature profile of the article during freezing, thus ensuring the quality of the frozen article.